This research concerns the mechanisms controlling the formation of the activated T cells that mediate delayed-type hypersensitivity and are the basis of cell-mediated immunity to infection. The mechanisms that regulate the formation of this cell type must be understood in order to design prophylactic procedures that will favor the induction of a T cell response when this is needed. Most immunizing regimens, with the exception of Freund's complete adjuvant or the use of living vaccine, result in antibody formation. Current evidence suggests that once antibodies have been produced it becomes impossible, because of feedback inhibition, to achieve an effective level of cell-mediated immunity. Most of the research for the ensuing year will be concerned with developing procedures that will favor the induction of T cells and minimize antibody formation. In practical terms, the objective is to find substitutes for live vaccines and a means of immunizing against parasites for which no suitable prophylactic procedures currently exist.